This invention relates to steels, and, more particularly, to an improved 1%CrMoV steel useful in producing large turbine rotors for electrical power generation.
Most of the electrical power produced by utilities is generated using steam or gas turbines. To produce the steam used in the steam turbines, water is vaporized in a heat exchanger heated by the burning of coal or petroleum, or through a controlled nuclear reaction. The steam is directed into the steam turbine, which has a series of turbine blades (also known as buckets) arranged around the periphery of a wheel or rotor. The rotor turns on a shaft under the impact of the steam against the turbine blades. The shaft is connected to an electrical generator, so that electrical power is generated as the shaft turns.
There are many types of specialty materials used in the construction of such turbines used for power generation. The present invention is directed to an improvement in one of these materials, the steel used in rotor forgings. The rotor of a steam turbine used in power generation, which may be as large as 200 centimeters diameter at its low pressure end, turns on its shaft at a rate of 3600 revolutions per minute (for 60 cycle power generation, the standard in the United States), and the high pressure end typically operates at a temperature of up to about 565.degree. C. These operating conditions are continued for thousands of hours in normal service. The material of construction of the rotor must be able to operate without failure under these conditions, with an acceptable margin of safety. The largest turbine rotors are among the largest one-piece forgings made in the world, and are very expensive. Improvements in the material of construction can have a major effect on the cost of electricity, the life of the powerplant between major overhauls, and the safety of the powerplant.
To meet these operating requirements, a specialty steel known as "1%CrMoV" steel has been developed and used over a period of many years. This steel has acceptable strength, creep resistance, resistance to notch sensitivity, and toughness, which are retained, at gradually reduced levels, during thousands of hours of operation of the rotor. The compositions of the 1%CrMoV steels are specified by ASTM standards A470 Class 8, as modified by supplements. Generally, the composition falls within the range 0.25-0.35 percent carbon, 1.0 percent maximum manganese, 0.15-0.35 percent silicon (with silicon below 0.10 percent permitted in some instances), 0.015 maximum phosphorus, 0.015 maximum sulfur, 0.75 percent maximum nickel, 0.90-1.50 chromium, 1.0-1.5 percent molybdenum, 0.20-0.30 percent vanadium, balance iron totalling 100 percent, with all percentages by weight.
Although the 1%CrMoV steel has proved to be highly effective in steam and gas turbine rotors now in service throughout the world, its use requires that special operating procedures be employed during startup and shutdown of the turbine. When the steel is given a conventional heat treatment of austenitizing at 950.degree. C., fan cooling (or oil quenching in European practice), and tempering to a tensile strength at 790 MPa, the 50% Fracture Appearance Transition Temperature (FATT) at the center of a typical 127 centimeter diameter forging is 90.degree. C., and the steel is potentially subject to temper embrittlement after long duration service exposure of elevated temperature loading and cooling during startup/shutdown cycling. Because the FATT is above ambient (room) temperature, the rotor must be prewarmed to a temperature above the FATT during startup, and must be carefully decelerated and cooled during shutdown, to avoid overload of the rotor in a temperature range of brittleness. These special startup and shutdown procedures lead to higher capital and fuel costs, and reduced operating flexibility for the utility. Conventional 1%CrMoV steel has lower hardenability than desired, and is susceptible to formation of ferrite at the center of large pieces.
It would therefore be desirable to identify a material of construction for steam and gas turbine rotors that retains the previously established and highly desirable characteristics and properties of the 1%CrMoV family of steels, but which has a reduced FATT, is more resistant to degradation in the form of decreasing mechanical properties and the appearance of temper brittlement, has better hardenability, and permits an extended design life of the rotor. The present invention fulfills this need, and further provides related advantages.